The functions of passing or dipped beam lights and foglights are examples of cut-off light beams conforming to current European legislation.
Generally, the cut-off in an elliptical headlight is produced using a mask which is in the form of a vertical plate the profile of which is suitably adapted and which is interposed axially between the elliptical reflector and the convergent lens, and which is arranged close to the second focus of the reflector.
The mask provides an occulting function for the light rays issued from the light source and reflected by the reflector to the lower part of the focal plane of the convergent lens. These rays would, in the absence of any mask, be emitted by the headlight above the cut-off line.
Such a solution does however have certain difficulties.
Thus, one disadvantage of this type of headlight is that a significant part of the light flux emitted by the light source is dissipated in the rear face of the mask.
Another solution consists in making a lighting module which employs a light source and a Fresnel lens or a reflector of the complex surface type. In order to create a cut-off line, it is necessary to align the edges of the images of the light source on the measuring screen which is used in carrying out adjustments to give a regulation light beam.
But again, this solution gives rise to some problems. In this connection, where the light source is a diode, it is very difficult to produce a full cut-off. This is because the image of the virtual source corresponding to the diode is generally round and is diffuse, and it is much more complicated to produce a clean cut-off line by aligning corresponding images of round forms.
This difficulty may be overcome by using a diaphragm with the diode, but a large quantity of the luminous energy produced by the diode is then lost.
In addition, the emission indicators of the known diodes having the best performance are complex, and it is very difficult to obtain a homogeneous beam from direct images of the diode.